These enable the system to do things automatically, such as turn on lights based on motion detection or raise the heating temperature just before your family arrives home so that your house is nice and warm when they open the front door. It enables you to add and configure devices and to create and run 'logic', sometimes referred to as flows, scenes or moods. As its name implies, a Smart Home 'Controller', sometimes called the hub, gateway or bridge, 'controls' your Smart Home.
You should start your Smart Home journey with the heart of the system - the Controller.
